Antistatic tyre having low-carbon black blends

Abstract: In a vehicle tyre provided with a tread band (9) made of a low-carbon black blend, one or more conductive inserts (12) of an elastomeric material having a high carbon black content are provided, which inserts are incorporated in the tread band and extend in an annular configuration along the whole circumferential extension of the tread band. Each conductive insert (12) passes through the whole tread thickness and is exposed externally of the rolling surface (9a) of the tread band itself, acting by its opposite side (12b) in contact relationship on the belt (6), the carcass ply (2) or another tyre part having sufficient electric conductivity to enable the electrostatic charges stored by the vehicle on running to be discharged to the ground through the tyre via the conductive inserts themselves.

Description: The present invention relates to an antistatic tyre the tread band of which has a low carbon black content, of the type comprising at least one carcass ply anchored, at respective opposite edges thereof, to a pair of bead cores arranged in corresponding beads defined along the inner circumferential edges of said tyre; at least one belt layer extending circumferentially about said carcass ply; a tread band disposed circumferentially about the belt layer and externally exhibiting a rolling surface designed to come into contact with the ground. It is known that vehicle tyres in general must have, in addition to usual roadholding and ride comfort features, also a certain degree of electric conductivity so that the electrostatic charges inevitably stored by the vehicle on running may be conveniently discharged to the ground, at least within some limits. In this connection, there are several different regulations for classifying the tyres depending on the electric resistivity they show and/or fixing the maximum resistivity limits that the tyres must exhibit based on the intended uses. By way of example, the German regulation of the WIRTSCHAFTSVERBAND DER DEUTSCHEN KAUTSCHUKINDUSTRIE E.V. (W.d.K. Leitlinie 110) classifies tyres into two different classes, identifying in the first class tyres having an electric resistance lower than 10 6 ohm, and in the second class tyres having an electric resistance lower than 10 8 ohm. Still by way of example, according to BSI (British Standards Institution) regulations BS 2050:1978 tyres used at the inside of hospitals must have an electric resistance value lower than 10 4 ohm. Still in accordance with these regulations, tyres used on industrial vehicles designed to carry explosives must not have an electric resistance higher than 5x10 5 ohm. In tyres of earlier conception, for example those disclosed in GB Patent No. 551 576, the necessary tyre conductivity was ensured by a particularly conductive rubber strip extending from the radially outer surface of the tread to the tyre bead, where it came into direct contact with the mounting rim located between the outermost carcass ply and the outer surface of the tyre. Substantially the same solution is described also in DE-C-820 543. GB-A-544 757 teaches that an electrically conducting rubber cement, when applied to specific portions of a pneumatic tyre, inside or outside the carcass body, or even inserted in said body, will function effectively in discharging static electricity. JP52-47202 teaches that, as an alternative to the electroconductive rubber tape connecting the bead portions of the tire, in contact whit the wheel rim, with an electroconductive rubber piece appropriately buried in the tread rubber, electroconductive rubber substances may be added to only some parts of the tire in order not to adversely influence the various properties required for the tyre. Such a solutions have not been in use for a long time and these were hardly used in the past because of the construction complications generated during the tyre manufacture and the problems relating to the qualitative features of the finished product. Consequently, in the tyres that are presently commonly used and commercialized, the requisite electric conductivity is ensured by the presence of the carbon black normally used as a reinforcing filler in the blends forming the different tyre parts. In this respect, the tyre manufacturers' efforts have been tendentially oriented towards researches aiming at finding out the modalities for use of carbon black in order to achieve the requisite electric conductivity values. For example, Patent FR 793,507 describes a blend made conductive by an appropriate selection of the characteristics of the carbon black introduced thereinto. Patent US 4,642,202 discloses a process for preparing conductive rubbers having a resistivity lower than 10 ohm*cm, consisting in adding to a normal blend formulation a composition formed of at least one material having a low compatibility with rubber and mixed with a highly conductive material such as some specific types of carbon black or metal powders. Patent US 5,173,135 describes a radial tyre for motor vehicles, the sidewalls of which are made of a blend consisting of an elastomeric material containing conductive black. In tyres of the above type the electrostatic charges stored in the vehicle bodywork on running are transferred to the tyre itself through the wheel rim and completely cross the tyre structure moving through the carbon black aggregates present in the polymeric matrix of the elastomeric material to be ultimately transferred to the ground at the contact area between the rolling surface of the tread band and the ground itself. Under this situation, the electric resistance value detectable on the above tyres can vary depending on the quantity and type of carbon black introduced into the blends forming the different parts of the tyre from a minimum of 10 2 ohm to a maximum of 10 6 ohm, and this value is therefore capable of complying with most requirements. It is pointed out that all resistivity and electric resistance values cited in the present description are intended as measured according to UNI 4288-71 Standard. The foregoing being stated, it is noted that tyres to be used in particular on motor-vehicles have been recently developed the tread band of which is made of a blend in which carbon black is containted in minimum or even negligible amounts, being for example completely or mostly replaced by silicon-based reinforcing fillers, such as silicon oxides or several different types of silicates for example. In the following such reinforcing fillers will be generally referred to as "silicon". The fundamental property of tyres thus conceived is a remarkably reduced rolling resistance as compared to the resistance that can be found in tyres with a tread band having a high carbon black content, in that the use of silicon-based reinforcing fillers instead of carbon-containing fillers gives the blend better hysteretic features. It has been found however that a carbon black reduction in the blend forming the tread band or a complete elimination thereof involves an important and on the other hand undesired increase in the electric resistance of the tyre, in that the conductive bridges supplied by the carbon present in carbon black are missing. The electrostatic charges produced in the vehicle on running therefore cannot be conveniently discharged to the ground and tend to be stored, thereby reaching very high potentials. Under this situation at the moment that the vehicle is touched by a person or an object creating a conductive connection between the vehicle and ground, the electrostatic energy previously stored is completely and suddenly discharged thereby producing an electric discharge of high potential, presently estimated in the order of 25 KV. On the contrary its intensity is deemed to be very low, in the order of milli-ampere. These electric discharges, besides being very bothersome when they occur to a person who is touching the motor-vehicle, also constitute a factor of great danger as regards supply operations taking into account the fact that the discharge can take place between the motor-vehicle bodywork and the fuel delivery gun. It is also to note that storage of the electrostatic charges in a running vehicle also gives rise to frequent electric discharges between different parts of the vehicle, causing unacceptable operating noise in the radio sets and/or any other electric or electronic apparatus installed on board of the vehicle. According to the present invention, it has been found that the electric resistance of a tyre with a tread band having a low carbon black content and high silicon (or other non conductive filler) content can be greatly reduced and brought to the requisite values if at least one conductive insert preferably extending over the whole circumferential extension of the tyre is associated with the tread band, said insert being such arranged and passing through the whole tread band thickness in such a manner that it performs the function of a conductive connection between the ground and one of the belt layers or the carcass ply, or a sidewall or another sufficiently conductive part of the tyre. The invention relates to an antistatic tyre, in particular having a low rolling resistance, characterized by the features of the second part of independent claim 1. Advantageously, the conductive insert exhibits an inner end that, depending on cases, can be arranged in contact relationship with the belt layer, the carcass ply or the elastomeric material forming one tyre sidewall, extending externally of the carcass ply from said bead to a corresponding side edge of the tread band. Preferably, the conductive insert extends over the whole circumferential extension of the tread band and, in one embodiment, is located at a centered position relative to the equatorial plane of the tyre and exhibits a width, measured parallelly of the tyre axis, included between 2 mm and 40 mm. In a second embodiment the tyre comprises at least two of said conductive inserts, located at symmetrically spaced apart positions relative to an equatorial plane of the tyre, each adjacent to one of the side edges of the tread band. In addition, at least at the rolling surface the extension of each conductive insert is provided to be broken by transverse cut-outs formed in the tread band to define respective blocks in said rolling surface. In this case the conductive inserts should be preferably associated with respective rows of mutually offset blocks so that at least one of the inserts is always in contact with the ground. In the presence of two or more conductive inserts, the sum of the insert widths measured parallelly of the tyre axis, is preferably included between 4 mm and 50 mm. Advantageously, the elastomeric material forming the conductive insert containing carbon black-based reinforcing fillers has an electric resistivity lower than 10 3 Kohm*m, whereas the elastomeric material forming the tread band containing silicon-based reinforcing fillers has an electric resistivity greater than 10 6 Kohm*m. At all events and to best advantage, the following proportion is to be taken into account: the greater the resistivity of the material forming the tread band is, the lower the resistivity of the blend forming the conductive insert. Further features and advantages will become more apparent from the detailed description of some preferred embodiments of an antistatic tyre made of blends having a low carbon black content, according to the present invention, given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: Fig. 1 is a fragmentary cross sectional view of one embodiment of a tyre according to the invention; Fig. 2 is a fragmentary cross sectional view of a second embodiment of the tyre in reference. Referring to the drawings, an antistatic tyre in accordance with the present invention has been generally identified by reference numeral 1. Conventionally tyre 1 comprises at least one carcass ply 2 the opposite side edges of which are folded up around corresponding anchoring bead cores 3, each incorporated in a bead 4 defined along an inner circumferential edge of the tyre at which the engagement of the tyre itself on a mounting rim 5 being part of a vehicle wheel takes place. Applied to the carcass ply 2 along the circumferential extension thereof is one or more belt layers 6 made of textile or metal cords incorporated in a blend sheet. Also applied to the external surface of the carcass ply 2 at respective opposite side positions thereof is a pair of sidewalls 7 each of which substantially extends from the bead 4 until the so-called "shoulder" region 8 of the tyre, defined in the vicinity of the end edges of the belt layers 6. To the ends of the present invention, the elastomeric material blends employed in making the sidewalls 7, belt layers 6 and carcass ply 2 respectively may advantageously be of a high-carbon black type, so as to exhibit an electric resistivity not exceeding 10 3 Kohm*m. A tread band 9 is circumferentially disposed around the belt layers 6, the side edges of said tread band terminating at the shoulders 8 and being connected to the sidewalls 7. The tread band 9 externally exhibits a rolling surface 9a arranged to act in contact relationship on the ground, in which surface circumferential grooves 10 alternating with the transverse cut-outs may be formed. Said grooves are not shown in the accompanying figures and they define a plurality of blocks 11 distributed in one or more circumferential rows 2. In the tyre 1 made in accordance with the present invention, the tread band 9 is made of an elastomeric material blend in which the carbon black, usually employed as the reinforcing filler in the remaining parts of the tyre itself, is completely or almost completely replaced by a silicon or another non conductive material adapted to improve the hysteretic features of the tread band and, as a result, reduce the tyre rolling resistance under use conditions. The presence of silicon in place of carbon black however involves an increase in the electric resistivity of the blend forming the tread band the value of which may easily exceed 10 6 Kohm*m. According to the present invention, problems resulting from the high resistivity of the low-carbon black blend forming the tread band 9 are solved by arranging at least one conductive insert 12 in the tread band itself, which insert is preferably made of an elastomeric material having an electric resistivity lower than that of the elastomeric material forming the tread band. The conductive insert 12 preferably extending according to the whole circumferential extension of the tyre 1, crosses the tread band 9 through the whole thickness thereof so as to exhibit one outer end 12a exposed to the outside which is flush with the rolling surface 9a in order to get into contact relation with the ground, so that the electric conductivity between the ground and the tyre will be increased. The conductive insert 12, on its side opposite to the outer end 12a has an inner end 12b that, depending on cases, can be arranged in contact relationship with one of the sidewalls 7 of the tyre or any other tyre part having an electric conductivity sufficient to ensure a good conductive connection between the ground and the rim 5 and therefore the vehicle bodywork, through the tyre 1. In order to ensure a good conductive connection between the tyre 1 and the ground through the tread band 9, the conductive insert 12 is provided to be made of elastomeric material-based blends having an electric resistivity lower than 10 3 Kohm*m, preferably achieved through the use of carbon black reinforcing fillers in said blends. Should the electric resistivity of the tread band be equal to or higher than 10 8 Kohm*m, the conductive insert would preferably have an electric resistivity lower than 100 Kohm*m or equal thereto. At all events, in general terms, the greater the electric resistivity of the parts of the tread band 9 other than the conductive insert 12, the lower the electric resistivity to be given to said insert. The Applicant deems it suitable to this end to observe the rule of coupling blends for treads and blends for conductive inserts the resistivity values of which, measured in Kohm*m, are such conceived that the algebraic sum of the respective exponents does not exceed 11; for example, in the above case where values are exactly 10 8 and 10 2 , one has 8+2=10. By modifying the axial size of the insert it will be then possible to give the tyre the desired electric resistance value minimizing the noise effect caused by the presence of the insert itself in the tread band. Referring particularly to Fig. 1, the example therein shown contemplates the presence of a single conductive insert 12 located at a centered position relative to the equatorial plane of the tyre 1. In this case, the conductive insert 12 preferentially has a width, measured parallel to the tyre axis, included between 2 mm and 40 mm. Lower or higher width values than the above could give rise to a weak reliability in the conductive connection with the ground or an excessive influence of the conductive insert on the tyre rolling resistance, respectively. Still referring to Fig. 1, the inner end 12b of the conductive insert 12 acts in contact relationship on the outermost one of the belt layers 6. Under this situation, the electric connection between the conductive insert 12 and rim 5 can advantageously take place through the metal cords optionally arranged in the belt layer 6 and carcass ply 2, or through the low-resistivity elastomeric material employed in making the plies themselves. We refer now to Fig. 2. In the tyre therein shown the presence of at least two conductive inserts 12 is provided and they are placed at symmetrically spaced apart positions relative to the tyre equatorial plane. In particular, each of the conductive inserts 12 is located adjacent to one of the side edges of the tread band 9 and its inner end 12b acts in contact relationship on the carcass ply 2 and/or the sidewall blend. Should the circumferential extension of each insert 12 be broken by effect of the presence of said transverse cut-outs in the tread band 9, it would be advantageous to envisage the association of the conductive inserts with corresponding rows of mutually offset blocks 11, so that, at each moment, at least one of the conductive inserts 12 is in contact relationship with the ground. In the above case, in order to ensure the continuous presence of this contact with the ground, it would be convenient for said inserts to be disposed, in respect of the type of tread pattern present on the band, in an asymmetric position relative to the equatorial plane of the tyre. Alternatively, the use of two conductive inserts disposed on the tyre shoulders symmetrically to the equatorial plane would be convenient, said inserts being in this case made of blends adapted to achieve an evener and more constant wear in these particularly critical tread areas due to the presence of high-silicon blends. Preferably, in the presence of two or more conductive inserts 12 the sum of the insert widths is provided to be in the range of 4 mm to 50 mm. The present invention achieves important advantages. The presence of the conductive inserts 12 in the tread band 9 in fact ensures that the vehicle bodywork on which the tyre in reference is mounted will be constantly connected to the ground, thereby eliminating any problem related to the use of tread bands made of low-carbon black elastomeric materials, as those having a high silicon content. Note should be also taken of the fact that the electric conduction carried out by utilizing the belt layers 6 and/or carcass layers 2 too, as well as the sidewalls 7 and any other tyre part, advantageously makes it possible to use conductive inserts the extension of which in a radial direction is tightly restrained to the tread band thickness without the necessity of using any additional elements for connecting them to the rim, as the presence of said elements in the tyre structure could undesirably alter the operating features thereof. It should be also recognized that, since the extension of the conductive insert or inserts is limited to the tread band thickness, said inserts can be directly associated with the tread band during the usual blend extrusion steps carried out for making the tread band itself, without requiring any additional working in the subsequent assembling steps of the different tyre parts. Obviously modifications and variation may be made to the invention as conceived, all of them falling within the scope of the invention, as defined in the claims.

Claims: A vehicle tyre for mounting on a rim, comprising: at least one carcass ply (2) anchored, at respective opposite edges thereof, to a pair of bead cores (3) arranged in corresponding beads (4) defined along the inner circumferential edges of said tyre (1); at least one belt layer (6) extending circumferentially about said carcass ply (2); a tread band (9) disposed circumferentially about the belt layer (6) and externally exhibiting a rolling surface (9a) designed to come into contact with the ground; means being provided for discharging to the ground the electrostatic charges stored on the vehicle through a conductive path formed into the tire structure; characterized in that said carcass ply (2) and belt layer (6) are conductive and in electric contact with each other; said tread band (9) is made of a filler including blend whose resistivity is not such as to constitute a conductive path from said rolling surface (9a) to said at least one conductive belt layer (6); said discharging means include at least one conductive insert (12) radially extending through the whole tread band (9) thickness, of a blend exhibiting an electric resistivity lower than that of the blend forming said band, to make a conductive path from said rolling surface (9a) to said at least one conductive belt layer (6). A tyre according to claim 1, characterized in that said conductive insert (12) exhibits an inner end (12b) in contact with said belt layer (6). A tyre according to claim 1, characterized in that said conductive insert (12) exhibits an inner end (12b) in contact with said carcass ply (2). A tyre according to claim 1, characterized in that said conductive insert (12) exhibits an inner end in contact with the elastomeric material forming one tyre sidewall (7), said sidewall extending externally of the carcass ply (2) from said bead (4) to a corresponding side edge of the tread band (9). A tyre according to claim 1, characterized in that said conductive insert (12) extends over the whole circumferential extension of the tread band (9). A tyre according to claim 5, characterized in that said conductive insert (12) is located at a centered position relative to the equatorial plane of the tyre (1). A tyre according to claim 5, characterized in that it comprises at least two of said conductive inserts (12) located on the opposite sides of said equatorial plane. A tyre according to claim 7, characterized in that said conductive inserts (12) are located at positions spaced apart symmetrically of the equatorial plane of the tyre (1), adjacent to the side edges of the tread band (9). A tyre according to claim 7, characterized in that at least at the rolling surface (9a), the extension of each conductive insert is broken by transverse cut-outs formed in the tread band (9) to define respective blocks (11) in said rolling surface, said conductive inserts (12) being associated with respective rows of mutually offset blocks. A tyre according to claim 5, characterized in that said conductive insert (12) has a width, measured parallelly of the tyre axis, included between 2 mm and 40 mm. A tyre according to claim 7, characterized in that the sum of the conductive insert (12) widths measured parallelly of the tyre axis, is included between 4 mm and 50 mm. A tyre according to claim 1, characterized in that the blend forming the conductive insert has an electric resistivity equal to or lower than 10 3 Kohm*m. A tyre according to claim 12, characterized in that the blend forming the conductive insert (12) contains carbon black-based reinforcing fillers. A tyre according to claim 13, characterized in that the blend forming the tread band (9) and the blend forming the conductive insert (12) have such respective electric resistivity values, measured in Kohm*m, that the algebraic sum of their exponents is not higher than 11. A tyre according to claim 1, characterized in that the blend forming the tread band (9) contains reinforcing fillers of non-conductive materials. A tyre according to claim 15, characterized in that said reinforcing fillers consist of silicon-based materials. A tyre according to claim 1, characterized in that said conductive insert (12) exhibits an inner end (12b) in contact with another part of the tyre other than the belt layers (6), the carcass layers (2) and/or the sidewalls (7). A tyre according to claim 1, characterized in that said inserts are associated with the tread band during the blend extrusion steps carried out for making said tread band itself. A tyre according to claim 1, characterized in that the extension of said insert (12) in a radial direction is tightly restrained to said tread band thickness. A tyre according to claim 15, characterized in that the blend forming the tread band (9) contains, as a reinforcing fillers, 100% of silicon based reinforcing fillers.

Inventors: BOIOCCHI MAURIZIO, NAHMIAS MARCO, BOIOCCHI, MAURIZIO, NAHMIAS, MARCO
Applicants: PIRELLI, PIRELLI PNEUMATICI SOCIETA PER AZIONI
Application year: 1994
Main classification: B60C19
IPC codes: B60C1, B60C11, B60C19